Coal has heretofore been used mainly in a powdered form for the commercial generation of heat energy. Such powdered coal, however, suffers from various problems; for example, it is difficult to transport, its combustion is difficult to control, its calorific value is low, it needs a large space for storage, and there is a danger of spontaneous ignition. In place of coal, therefore, heavy oil has been increasingly used as an energy source.
In recent years, however, in view of problems such as exhaustion of fuel oil and a steep rise in its price, coal has again received increasing attention.
Various attempts to overcome the above-described problems of coal by mixing it with oil were made before World War II as described in, for example, German Pat. Nos. 637,437 and 638,662 (1936). However, when common powdered coal is merely mixed with oil, coal particles will precipitate, forming a solid phase having no fluidity due to the difference in specific gravity between coal and oil. Therefore, it is difficult to store the mixture in a stabilized condition over a long period of time.
In order to improve the storage stability and fluidity of mixed fuel comprising powdered coal and oil, an attempt has been made to further reduce the size of powdered coal particles, so that part of the powered coal particles are capable of forming a colloid (see Japanese Patent Application (OPI) No. 40808/79 (the term "OPI" as used herein means a "published unexamined Japanese patent application")). It has also been proposed that such a super-finely powdered coal is not mixed with oil but suspended in water, and the resulting powdered coal-water slurry is transported and burned (see published unexamined PCT Patent Application in Japan No. 501568/81).
Production, however, of such powdered coal in a super-finely powdered form capable of forming a colloid requires a large quantity of energy and an expensive complicated apparatus for pulverizing the coal. Furthermore, there is a substantial danger of so-called dust explosion when producing such super-finely powdered coal before mixing it with oil or water. In view of such practical problems, the methods have not yet been performed on a commercial scale.
Studies to produce stabilized mixed fuels by mixing common powdered coal with oil as such, i.e., without super-finely pulverizing coal, have been made. Various mixed fuels have thus been proposed wherein water and a dispersion stabilizer are incorporated into such a mixed system of powdered coal and oil to form a network structure of oil/dispersion stabilizer/water/dispersion stabilizer/powdered coal.
Dispersion stabilizers proposed for use in the formation of such network structures are, as can be anticipated by the stabilization mechanism based on the network structure, water-soluble organic compounds and organic polymer compounds which have surface activity or thickening properties. Examples of such compounds include anionic surface active agents, e.g., alkylbenzenesulfonic acid salts and mono- or poly-carboxylic acid salts (Japanese Patent Application (OPI) Nos. 82809/78, 82811/78, etc.), amine-based cationic surface active agents, e.g., mono- or di-alkyl quaternary ammonium salts, mono- or poly-amines and their derivatives, and amines containing an amido bond or an ether bond (Japanese Patent Application (OPI) Nos. 82810/78, 82807/78, etc.), nonionic surface active agents, e.g., polyethers or polyetherpolyols having molecular weights of from 1,000 to 100,000, derived from ethylene oxide, propylene oxide, or the like, and their cross-linked derivatives (Japanese Patent Application (OPI) Nos. 52105/79, 53105/79, 52106/79, etc.), and water-soluble polymeric compounds, e.g., carboxymethyl cellulose, carboxyethyl cellulose, carboxymethyl starch, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, polyethylene glycol cellulose ether, cellulose acetate, and natural gums, e.g., as guar gum, locustbean gum and alginic acid (Japanese Patent Application (OPI) No. 50203/78).
These dispersion stabilizers, however, are water-soluble organic compounds or organic polymeric compounds, or compounds derived from natural polymeric compounds. Therefore, it is necessary to add them in high proportions. For example, in the mixed fuel of powdered coal and heavy oil C (defined by Japanese Industrial Standard (JIS) K2205 (1958); heavy oil having flash point of at least 70.degree. C., viscosity .eta..sub.50.degree. C. of 150 cps or less and pour point of 15.degree. C. or less) which is most inexpensive among fuel oils and is widely used, the amount of the dispersion stabilizer added is as high as about 1%. In order to reduce the amount of the dispersion stabilizer to from 0.1 to 0.3%, it is necessary to decrease the ratio of powdered coal to oil to less than 1/1, or alternatively, to increase the amount of water added to from 2 to 20%. This is disadvantageous from an economic standpoint and, further, gives rise to the problem that polymeric ones of the water-soluble dispersion stabilizers seriously increase the viscosity of the system.